Large-diameter steel pipes used to construct pipelines are primarily high-strength welded steel pipes in order to reduce their construction costs. In existing pipelines, high strength welded steel pipes of API X70 grade are mainly used, while higher strength API X80 grade welded steel pipes have been actually used in only a few instances. However, in recent years, the use of ultrahigh strength welded steel pipes which have not existed up to now such as pipes corresponding to so-called API X100 grade or API X120 grade has been investigated for use in pipelines. At present, such ultrahigh strength welded steel pipes have not been standardized as steel pipes for line pipe, but there is a high probability that they will be formally standardized in the near future.
In the following explanation, the expressions “corresponding to so-called API X100 grade” and “corresponding to so-called API X120” will be abbreviated as “X100 grade” and “X120 grade”. The standards for X100 grade are expected to include a yield strength YS of at least 690 MPa, a tensile strength TS of at least 760 MPa, a yield ratio YR of at most 97.0%, and a Charpy absorbed energy at −10° C. of at least 80 J for the base metal, as well as a Charpy absorbed energy at −10° C. of at least 40 J and a shear area of at least 50% for the weld metal, and Charpy absorbed energy at −10° C. of at least 40 J and a shear area of at least 50% for the weld heat affected zone.
As the strength of welded steel pipe for line pipe reaches an ultrahigh level, it is very likely that an ultrahigh strength of at least X100 grade will also be demanded of bent pipes, which are indispensable in the construction of pipelines. However, satisfactory manufacturing techniques for ultrahigh strength bent pipes have yet to be established. This is because it is difficult to achieve a high level of strength and toughness in a bent pipe by heat treatment, which is indispensable in the manufacture of a bent pipe.
A large number of inventions relating to high strength bent pipes have been proposed in the past. See, for example, JP H07-3330 A1, JP H08-92649 A1, JP 2003-277831 A1, JP 2004-332083 A1, and JP 2005-350724 A1. These documents disclose inventions in which a high strength bent pipe is manufactured by prescribing the composition of a bend mother pipe which is a straight steel pipe before bending as well as the manufacturing conditions for the bent pipe. However, these inventions do not take into consideration the manufacturing conditions of the mother pipe or the steel plate used to form the mother pipe.